1. Field of the Invention
The technical scope of the present invention is that of double wall pipes comprising an inner pipe and an outer pipe between which a heat insulator has been inserted and more particularly means to link two sections of piping.
2. Description of the Related Art
The piping in question relates to double wall tubing enabling the transportation of fluid and installed in a well reinforced by casing pipes. These pipes are generally manufactured in the form of sections of pre-determined length, 8 to 12 meters, for example, which must be connected together. The problem arises thereafter of the thermal insulation at the junction of two consecutive tubings. Indeed, it is important to avoid any cooling of the relatively hot fluid from the deep deposit and the ambient temperature of the surface or any heat exchange during the injection of vapor. In this case, the purpose of the thermal insulation is to preserve the quality of the vapor and to protect the casing pipe from excessive heat or else to modify the thermal balance of the subsoil. Among the negative effects of any cooling of the crude oil in this type of piping, we can mention the following points:                paraffin deposits,        asphaltene precipitation,        hydrate formation,        heat transfer with the casing pipes thereby damaging said casing pipes.        
These sections are generally connected by screwing the inner pipes according to two main principles: screwing together male-female ends or screwing together male-male ends using a fitting female-female generally named coupling.
Given the reduced amount of space available and the high pressures in a well, we are obliged to look towards highly effective thermal insulation materials such as those developed for use in space or cryogenics, using vacuum-based technologies and anti-radiative screens.
In practice, we observe that the heat losses at the junction of two sections are easily ten times greater than those of the sections themselves. Thus, for an exchange coefficient for a single section of around 0.1 W/m2/K, this coefficient is one thousand times greater at the junction. Thus, if the coefficient is of 0.1 W/m2/K in the continuous sections, globally with the existing technologies it is very difficult to obtain better than 2.5 W/m2/K.
Thus, to overcome this drawback, a cylindrical insulation material has been proposed, made of a plastic material and installed at the connector. These materials are made using dense materials which are therefore not very insulating. However, this technique does not enable the use of good quality insulation materials, such as honeycomb structure materials, for example polypropylene foam with the inclusion or not of glass micro-beads. By way of example, we observe that a 15 mm cylinder made of solid polypropylene provides heat insulation of around 10 to 30 W/m2/K compared with the 0.1 W/m2/K obtained by a vacuum insulation of the same thickness.
We note the great importance of the problem of thermal insulation at the connectors of two consecutive sections which may considerably reduce the thermal performances of such a pipeline.